Objective: The long term goal of this project is to determine the structural features of the active site of Cl esterase enzyme. This particular enzyme is part of the macromolecular complex comprising Cl, the first component of human complement. The implications of such information will be explored by examining the active site of Cl esterase enzyme isolated from both normal and diseased states. Abstract: It is proposed to investigate the structural features of human Cl minus s, the esterase enzyme which constitutes one-third of the macromolecular complex, Cl, the first component of serum complement. Procedures to be used in this study include molecular weight analyses, carboxy-terminal and amino-terminal amino acid analysis of the protein, overall amino acid analysis, and determination of individual amino acids and peptide segments involved in the function of the esteratic site. The latter information will be obtained using the technique of affinity labeling, by which a reagent is specifically and covalently bound to some portion of the active site. Finally, it is hoped to characterize Cl minus s isolated serum and gut tissue samples. The Cl minus s in such samples will be isolated and purified by affinity chromatography, a purification procedure which results in almost theoretical yields of the Cl minus s enzyme from serum. The information gained from these studies will be useful in terms of developing new methods for the investigation of the function and structure of other complement proteins, in providing insights into possible mechanisms by which the complement proteins generate specificity for each other, and in understanding the way in which the complement system participates in host defense mechanisms.